Paul Holmes, Unison’s Kirklees branch secretary, explained that members were “at the end of their tether.”

He added: “The issue is that the council is not trusted by the men and now they want to see proof,” he said.

“I spoke to one bloke who has 40 weeks of holiday he hasn’t been able to take. He says, ‘it’s all right them telling me what they’re going to do about it but I need to see a signed holiday card.’

“People say ‘why are the bins not being emptied’ and these guys are saying ‘hang on, I’ve spent 40 weeks of my holidays emptying them’.

“They all do it all their lives. They are at the end of their tether.”

The council has already promised the results of an investigation into alleged bullying, harassment and racism will be released at the beginning of this week – around the same time members go back to work.

“From our point of view the timing works out perfectly,” said Mr Holmes. “By the time the 17th comes around we’ll know the outcome of the investigation and also what the council propose to do about holiday and sick pay.

“We can assess what they have done and make a decision on whether we need to strike.”

Mr Holmes said council actions during the first day of the current strike had changed the mood among the union members.

A picket line at the Vine Street Depot in Huddersfield on Tuesday morning was met by a number of police officers and dog units.

“Two things happened. First, they brought dogs to the picket line and I’ve not seen that since the miners’ strike.

“At least in the miners’ strike they were police dogs. Here they were private security with dogs and it wasn’t like it was an aggressive protest. It was placards and bin workers sat in deck chairs.

“Second, the council illegally blocked off Emerald Street, which is a public road. One of our officers tried to get down there and was stopped by security with two dogs.

“Imagine if I went out into the street and blocked it to the public – the police would be called.

“So the workers have lost all trust in the council. The mood is high.”

Karl Battersby, Strategic Director Economy and Infrastructure said: “A security guard was on duty overnight to ensure the security of the vehicles that had been moved for operational reasons. As part of his duties he was accompanied by a dog.

"We are not aware of any police dogs being present and did not request the presence of dogs at the picket line.”